If the south peacefully resigned from the Union, would the North still have taken military action? Was there anything legally holding the Union together, could the states have just asked for a divorce?
The cause of the Civil War - the refusal of Southern states to give up the right to enslave human beings - is laid out in detail in this older thread by /u/Georgy_K_Zhukov. The legality of secession is a difficult subject which /u/freedmenspatrol covers in detail here.